A variety of types of speed changing mechanisms have long been known in the art. Among these types are those generally described as epicyclic, wherein at least one gear is made to undergo orbital as well as rotational motion. It has long been known that such types of gearing are very effective as speed reducers, as a large ratio speed reduction may be obtained by means of a relatively small gear train arrangement.
In order to obtain the maximum speed reduction possible, prior art epicyclic gearing systems have employed meshing gears wherein the number of teeth on a "ring" gear and that of the "orbiting" gear differed by one. While such a system was quite effective for torque transmission, as a number of teeth were simultaneously engaged with one another, this type of gearing was very expensive to manufacture, because the machining tolerances of the gears and of the individual teeth were quite critical.
Furthermore, as a drive for such epicyclic gearing, a shaft with an eccentric lobe resembling a cam was normally provided, this part normally being machined out of a single piece of stock. Such a driving means, of course, was quite expensive to produce, and even more so if the shaft was integrally provided with counterweights. Moreover, all known prior epicyclic gear trains require a number of conventional bearings located at the input and output of the gear box, as well as at intermediate locations within the gear train itself. Of course, the inclusion of such bearings greatly added to the cost of the overall device.